XF 1.2 Missing threads for some users

We have this problem where just from time to time, a user will report that a thread is missing from forum view. It's hard to say how widespread it is because a user may not be aware of the issue. The users concerned aren't anybody special, eg recently banned, discouraged or had any other thing happen to them as far as I am aware.

For example, this is how one particular forum should look

but what the user sees is below:

As you can see the top thread is missing.

The user can see the thread via Tapatalk. The thread isn't being watched. This particular user seems to have a history of this happening to him, he has 350 threads in his watch list. I've also tested his permissions via the admin panel and have not been able to reproduce the issue.